Commercially available polylactide at present is mostly poly(L-lactide) (hereinafter referred to as PLLA) and is synthesized from L-lactic acid with high optical purity obtained by the fermentation of a farm product (for example, starch). In order to synthesize PLLA from L-lactic acid, a method in which first L-lactide (hereinafter referred to as LLA) which is a cyclic dimer of L-lactic acid is synthesized, followed by the ring-opening polymerization of LLA as shown in the following chemical formula is generally used (Patent Literature 1). In order to synthesize crystalline PLLA, the optical purity of LLA to be used as a starting material is crucial. PLLA acquires a crystalline structure in which the PLLA molecules are regularly arranged. However, it is known that polylactides (PLA) synthesized from LLA with ≧8% of D-lactide (DLA) and/or meso-lactide, which are an optical isomers of LLA, are amorphous because the PLA molecules cannot be regularly arranged (Non Patent Literature 1).

In contrast, there is a known method in which semi-crystalline PLA is synthesized by the polymerization of a mixture of equivalent amounts of LLA and DLA, in other words, racemic lactide (rac-LA) with a stereoselective salen-aluminum catalyst. One example of the stereoselective salen-aluminum catalyst is shown in the following formula. In the formula, R is a bulky substituent such as a phenyl group, a tert-butyl group, or a tert-butyldimethylsilyl group.

The stereoselective mechanism of the catalyst is shown in the following chemical formula. The catalyst which is first coupled to LLA selectively activates LLA after coupled thereto and permits the ring-opening polymerization thereof, whereby a poly(L-lactide)-block (PLLA-block) is grown. On the other hand, the catalyst which is first coupled to DLA selectively activates DLA after coupled thereto and permits the ring-opening polymerization thereof, whereby a poly(D-lactide)-block (PDLA-block) is grown. The salen-aluminum catalyst can selectively polymerize LLA and DLA in this manner, and therefore, the resulting poly(racemic lactide) is composed of PLLA blocks and PDLA blocks. The poly(racemic lactide) obtained by this method acquires a semi-crystalline structure by forming a stereocomplex in which the poly(racemic lactide) molecules are regularly arranged (Patent Literature 2).

As the isomers of lactide, other than LLA and DLA, there is also meso-lactide (meso-LA). The salen-aluminum catalyst can selectively polymerize LLA and DLA, however the ability of the salen-aluminum catalyst to selectively polymerize a LLA, DLA, and meso-LA mixture is not high. For this reason, even if the salen-aluminum catalyst as shown in the patent shown previously (Patent Literature 2) is used, PLA obtained from a mixture of rac-LA and meso-LA is a polylactide resulting from the random polymerization of LLA, DLA, and meso-LA. In such polylactide, the polylactide molecules are not regularly arranged and a stereocomplex cannot be formed, and therefore semi-crystalline PLA cannot be obtained.